Air pollution and anthropogenic aerosol are typical byproducts of combustion, such as from automobiles, power plants, factories, fire places, and the like. Incomplete combustion often produces an aerosol that strongly absorbs visible light and has significant elemental carbon content. Not only do these aerosols negatively impact visibility, they may be a health hazard when inhaled and may alter the global radiation balance and general circulation. Common windborn dust from deserts also absorbs light at UV wavelengths.
A number of techniques have been devised to measure light absorption by aerosols. Many techniques capture aerosols on filters, followed by an optical measurement to determine aerosol light absorption. The Aethalometer is a real time version of such an instrument. However, the use of filters may alter particles in the aerosol and therefore the data obtained from such instruments may not accurately represent the natural state of the aerosol. Accordingly, these techniques may produce data which does not accurately describe the aerosol.
Photoacoustic techniques have also been used to measure the properties of aerosols. In the photoacoustic technique, electromagnetic energy is applied to a sample, some of which is absorbed by the aerosol and converted to heat. Because the particles in the aerosol are small, and have sufficiently high thermal conductivity, the absorbed heat will flow rapidly to the surrounding air. When heated, the air expands its volume or pressure. By placing the aerosol laden air into an acoustic resonator and modulating electromagnetic power at the resonance frequency of the acoustic resonator, the varying pressure disturbance (acoustic signal) can be amplified by the buildup of a standing acoustic wave in the resonator. Thus, by measuring the sound pressure associated with aerosol light absorption, a measure of elemental carbon concentration can be obtained.
Typical photoacoustic instruments measure light absorption. However, it is often necessary to measure extinction, which is the sum of absorption and scattering due to interaction of the particles with the electromagnetic radiation, to obtain more information on the climate impact of aerosols. One method of measuring extinction uses a laser as the light source and measures optical power before and after the introduction of an aerosol sample. The noise floor of such a measurement, however, can be very large (103 Mm−1) while the noise floor of the scattering and absorption measurements are less than 1 Mm−1 for comparable measurement times. Although cavity-ringdown extinction measurements have been combined with scattering measurements, these methods are still inadequate for many applications that require light absorption measurements.
For example, wood smoke from smoldering fires can have a single scattering albedo (ratio of scattering and extinction) of 0.99, so the extinction and scattering measurements must be unrealistically accurate and precise to obtain absorption by subtraction. Extinction measurements are generally plagued by imperfections in the amount of forward-scattered light reaching the detector in excess of the on-axis light, and scattering measurements may suffer from angular truncation errors.
For example, a readily available scientific nephelometer, manufactured by TSI, records particle scattering from 7 degrees in the forward direction to approximately 173 degrees in the backward direction. Scattering from submicron particles can be corrected by use of the Angström coefficient; however, such an approach for super-micron particles can produce large errors as described in Anderson, T. L. and J. A. Ogren (1998). “Determining Aerosol Radiative Properties Using the TSI 3563 Integrating Nephelometer” Aerosol Science and Technology 29(1): 57-69.
Another limitation of many photoacoustic instruments is that they are only capable of monitoring one wavelength at a time. As a result, prior techniques often require multiple instruments, or multiple sequential experiments with one instrument, in order to obtain information about multiple aerosol components or characteristic of an aerosol component. For example, more than one measurement may be needed to determine both the amount of particulate matter in an aerosol and to get information on the aerosol's composition, such as coatings on the particles.